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Breaker & Wire Size Compatibility Checker

A breaker that matches the ampacity table can still be wrong: the small conductor rule, the termination temperature, the conductor count in the raceway and the load itself each impose their own limit. This tool runs every rule separately and tells you which one failed and what to change, rather than returning a single verdict with no explanation.
Code basis
NEC 2023, Art. 240 / 310
Editions
2017, 2020, 2023
Checks run
Up to 6, reported separately
Verdict
Pass or fail per rule
Conductor sizes
14 AWG – 1000 kcmil
Materials
Copper, aluminum
Refuses
Motor, HVAC, welder

Circuit type

Motor, air-conditioning, welder and transformer circuits follow different rules and are refused rather than answered.

What does this circuit supply?

Breaker and conductor

Enter what is installed or what you intend to install. Non-standard ratings are flagged rather than rejected.

The overcurrent device ahead of the conductor.

Read from the 90 °C column.

The lower-rated end of the two. Equipment rated 100 A or less is 60 degrees C unless marked otherwise.

Optional. Adds a check against the table minimum.

Conditions and load

Conditions decide the conductor's real ampacity. The load decides whether the breaker is large enough.

Air around the raceway, not the room thermostat.

Excludes the grounding conductor and a neutral carrying only unbalanced current.

Optional. Leave blank to check the breaker against the conductor only.

Match the edition your jurisdiction enforces.

Largest permitted breaker

Compatible

30 A

Entered: 30 A on 10 AWG copper, 90 °C insulation into 75 °C terminations.

Checks passed
4 of 4
Conductor ampacity here
30 Aafter every correction and cap
Smallest breaker for the load
25 A24 A required
Grounding conductor
10 AWGminimum for this device rating
  • A 30 A device requires 10 AWG copper as a minimum grounding conductor. Select the size installed to have it checked.
Size a conductor from scratch instead

Every check, individually

  • Breaker is a standard rating

    Pass

    30 A is a standard rating.

    240.6(A)

  • Conductor is protected at its ampacity

    Pass

    The conductor is good for 30 A under these conditions, so a 30 A breaker protects it.

    240.4(B)

  • Within the small conductor limit

    Pass

    10 AWG copper is capped at 30 A, and 30 A is within it.

    240.4(D)

  • Breaker covers the load

    Pass

    A 24 A load is within the 30 A breaker.

    210.20(A)

Conductor ampacity

StepBasisEffectValue
Base ampacity, 10 AWG copperTable 310.16—40 A
Termination rated 75 C110.14(C)cap 3535 A
Small conductor overcurrent limitGoverning240.4(D)cap 3030 A
Governed by Small conductor overcurrent limit (240.4(D)), not by the load itself.
Derivation of what the entered conductor is good for under the entered conditions

The tabulated value is the starting point. Everything below it is a correction, an adjustment or a cap that the breaker rating has to respect.

Calculated against the NEC 2023 edition using Table 310.16, 110.14(C), 240.6(A), 240.4(B), 240.4(D), 210.20(A), Table 250.122. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Motor, air-conditioning, welder and transformer circuits, which are refused rather than answered.
  • Tap conductors, transformer secondary conductors and the other exceptions to the small conductor rule.
  • Receptacle rating against branch-circuit rating, which is a separate check on the device rather than on the conductor.
  • Ground-fault and arc-fault protection requirements, which are set by the area served rather than by the conductor.
  • Series-rated and current-limiting combinations, and available fault current at the equipment.
  • Conductors in free air, which are sized from a different ampacity table.

Data last verified

What Gets Checked

Six rules, evaluated independently. A pass on one is not a pass on the others.

  1. Check 1

    Standard device rating

    Confirms the breaker is a rating that is actually manufactured and listed. A non-standard rating is permitted in limited cases but is rarely what was intended, so it is reported as a caution rather than a failure.

  2. Check 2

    Conductor protected at its ampacity

    Compares the device rating against the conductor's ampacity after every correction and cap, applying the next-standard-size-up allowance where the circuit qualifies for it.

  3. Check 3

    Small conductor limit

    Enforces the fixed overcurrent ceilings on 14, 12 and 10 AWG. These override the ampacity table entirely, which is why a 12 AWG conductor with a tabulated 30 A ampacity still cannot be protected above 20 A.

  4. Check 4

    Terminal temperature

    Caps the conductor at the ampacity in the column matching the lowest-rated termination. Surfaced whenever that cap is what held the number down, because reading the 90 degrees C column alone is the most common way to reach a wrong answer here.

  5. Check 5

    Breaker covers the load

    Checks the device against the operating current, with the 125 percent multiplier applied when the load is continuous. A breaker can be too small as easily as it can be too large.

  6. Check 6

    Grounding conductor size

    Compares the installed equipment grounding conductor against the minimum for the device rating. Optional, because the field is frequently unknown at the point someone is checking a breaker.

Every rule is reported on its own line with the numbers that decided it. Knowing the conductor is fine and only the small conductor rule is violated tells you to change the breaker; knowing only that the combination failed tells you nothing.

Breaker Ceiling by Conductor Size

Copper into 75 degrees C terminations, in benign conditions. Correct these downward for ambient temperature and conductor count.

SizeAmpacity 75 °CMax breakerMin. EGCTypical circuit
14 AWG2015 A14 AWG15 A lighting and receptacle circuits
12 AWG2520 A12 AWG20 A general-purpose and kitchen circuits
10 AWG3530 A10 AWG30 A dryer, water heater, small subpanel
8 AWG5050 A10 AWG40 to 50 A range, EVSE, subpanel feeder
6 AWG6560 A10 AWG50 to 60 A range, hot tub, subpanel feeder
4 AWG8580 A8 AWG70 A subpanel feeder
3 AWG100100 A8 AWG85 A feeder
2 AWG115110 A6 AWG90 to 100 A subpanel feeder
1 AWG130125 A6 AWG100 to 110 A feeder
1/0 AWG150150 A6 AWG125 A feeder
2/0 AWG175175 A6 AWG150 A feeder
3/0 AWG200200 A6 AWG175 to 200 A feeder
4/0 AWG230225 A4 AWG200 A feeder
Largest overcurrent device each copper conductor may be protected by at 75 degrees C terminations before any correction, with the matching minimum grounding conductor. Derived and reorganised from the NEC ampacity, standard device rating and grounding conductor tables; consult the published code for the authoritative values.

These are ceilings in ideal conditions, not answers. An attic at 130 degrees F removes a quarter of the ampacity, and nine conductors in one raceway remove thirty percent. Both are common, and both move the numbers above.

Full breaker and wire chartGrounding conductor table

Common Combinations and Their Verdicts

The pairings that come up most, and the condition that flips each one.

CombinationVerdictGoverning rule
30 A breaker on 10 AWG copperPassAt the small conductor ceiling exactly. Correct for a dryer or water heater in normal ambient with three conductors in the raceway.
20 A breaker on 14 AWG copperFail14 AWG cannot be protected above 15 A regardless of its tabulated ampacity. The most frequent real-world violation this tool catches.
50 A breaker on 8 AWG THHN into 75 degrees C lugsPassThe 90 degrees C column reads 55 A, but the terminations cap it at 50 A, which the device matches exactly.
60 A breaker on 6 AWG NM cableFailCable assemblies are limited to the 60 degrees C column, where 6 AWG is 55 A. The same conductor in conduit as THHN passes.
40 A breaker on 8 AWG with nine conductors in the racewayFailThe bundling adjustment removes 30 percent of the ampacity. The conductor is compliant on its own and fails in company.
100 A breaker on 4 AWG copper feeding a subpanelFail4 AWG is 85 A at 75 degrees C. The next-size-up allowance reaches 90 A, not 100 A. Use 3 AWG or larger.
Frequently checked breaker and conductor combinations with the rule that decides each.

Circuits This Tool Will Not Judge

Four cases where a general branch-circuit check produces a confidently wrong answer. The tool refuses them instead.

  • Motors and air conditioning

    Branch-circuit protection is selected from the motor tables at a rating normally well above the conductor ampacity, because it guards against short circuit and ground fault while a separate overload device protects the conductor. An ampacity comparison returns a fail on a compliant installation.

  • Equipment marked with a minimum circuit ampacity

    Air conditioners, heat pumps, mini splits and refrigeration equipment carry a marked minimum circuit ampacity and a marked maximum overcurrent device rating. Those markings govern. Size the conductor to the minimum circuit ampacity and set the device at or below the marked maximum.

  • Welders and transformers

    Welder conductors are sized from rated primary current and duty cycle, with overcurrent devices permitted at multiples of that current. Transformer protection depends on what secondary protection is provided. Neither reduces to a conductor-ampacity comparison.

  • Tap conductors and secondary conductors

    Tap rules permit a conductor to be protected at a rating above its own ampacity under stated length and termination conditions. This tool applies the general rule, so a legitimate tap will read as a failure here.

What to Change When a Check Fails

Each failure has a specific remedy. Work from the rule that failed, not from the verdict.

  • The conductor fails on ampacity

    Either fit the largest device the conductor permits, shown in the result panel, or increase the conductor. Increasing insulation temperature rating rarely helps on its own, because the terminations usually cap the result before the insulation does.

  • The small conductor rule fails

    There is no way around it on 14, 12 or 10 AWG. Reduce the device to the stated ceiling or move up a size. The exceptions in that rule cover tap and motor applications, not general branch circuits.

  • The termination temperature is the cap

    Check the equipment label before assuming 75 degrees C. Terminations on equipment rated 100 A or less are treated as 60 degrees C unless marked otherwise, which is what turns a compliant-looking 8 AWG at 50 A into a failure on some panels.

  • The breaker does not cover the load

    Move up to the next standard rating and re-check the conductor against it. A continuous load needs 125 percent of its current available, so a 40 A continuous load requires 50 A of device and a conductor to match.

  • Conductor count triggered an adjustment

    Split the circuits between raceways, or accept the derated ampacity and size up. Counting is where most errors start: three two-wire circuits in one raceway is six current-carrying conductors, and the grounding conductor is never counted.

  • The grounding conductor is undersized

    Size it from the device rating, not from the ungrounded conductors. Where the ungrounded conductors were upsized for voltage drop, increase it proportionally to their increase in circular mil area.

FAQs

Is a 30 amp breaker safe on 10 AWG wire?
For a general-purpose copper branch circuit in normal conditions, yes. 30 A is exactly the small conductor ceiling for 10 AWG copper, and the tabulated ampacity at 75 degrees C is 35 A, so the conductor is protected. It stops being correct once conditions change: at a 110 degrees F ambient, or with more than three current-carrying conductors in the raceway, the corrected ampacity falls below 30 A and the combination fails. Enter the actual conditions rather than relying on the general answer.
Why does 12 AWG fail on a 30 amp breaker if the table says 30?
Because the small conductor rule sets a fixed overcurrent ceiling of 20 A for 12 AWG copper that overrides the ampacity table. The 30 A figure is the 90 degrees C tabulated ampacity, which is used for derating arithmetic, not for selecting the device. The rule exists because these conductors are the ones most exposed to physical damage and to being loaded in ways the table does not anticipate.
When can a breaker be larger than the conductor's ampacity?
Where the conductor's ampacity does not correspond to a standard device rating, the next standard rating above it may be used, provided the circuit supplies no receptacle outlets and the rating does not exceed 800 A. A 4 AWG copper conductor at 85 A may therefore be protected at 90 A on a feeder, but not on a receptacle circuit. Tick the receptacle box in the calculator and watch the permitted device drop.
Why is this tool refusing to check my mini split?
Because it would have to return a wrong answer. Motor and air-conditioning branch circuits are protected at ratings taken from the motor tables, commonly well above the conductor ampacity, since the device covers short circuit and ground fault while a separate overload device protects the conductor. Marked equipment is more direct still: size the conductor to the minimum circuit ampacity on the nameplate and set the breaker at or below the marked maximum overcurrent device rating.
Does the breaker brand or type matter?
Not to these checks. All of them concern the ampere rating and the conductor. Type matters elsewhere: the breaker must be listed for the panelboard it goes into, the interrupting rating must equal or exceed the available fault current, ground-fault and arc-fault types are required by the area served, and terminals must be listed for the conductor material. Substituting a brand not listed for the panelboard is a violation independent of any sizing question.
Do I enter the breaker size or the load?
Both, where you know them. The breaker rating and conductor are enough to check protection, the small conductor limit and the terminations. Adding the load current checks the other direction, that the device is large enough for what it serves, and applies the 125 percent multiplier if the load is continuous. Leave the load blank and those checks are skipped rather than guessed.
Why does the same conductor pass in conduit and fail in cable?
Cable assemblies such as NM are limited to the 60 degrees C ampacity column regardless of the temperature rating of the conductors inside them. 6 AWG copper is 55 A at 60 degrees C and 65 A at 75 degrees C, so the same conductor supports a 60 A device as THHN in conduit and does not as NM cable. Select the insulation that matches what is actually installed.
Does this tell me the circuit is code compliant?
No. It checks the specific rules listed on this page against the values you entered. A compliant circuit also depends on the raceway, the wiring method, the box fill, the receptacle rating, ground-fault and arc-fault protection, available fault current, working clearances and the terminations being listed for the conductor. Final determination rests with the authority having jurisdiction.